The present invention relates generally to digital-to-analog converters, and more particularly, to digital-to-analog converters using sigma-delta modulators.
A preferred technique for achieving accurate analog conversion of digital signals in digital-to-analog converters (DACs) is the noise-shaping technique. This technique makes use of digital signal processing to reduce the required accuracy and complexity of analog components, and by reducing the difficulty of design of these analog components. A noise-shaped sigma-delta DAC includes a digital sigma-delta modulator which quantizes an oversampled digital input signal to a small number of levels. The digital sigma-delta modultor shapes the quantization noise out of the passband of interest. The quantized digital output signal is converted to an analog signal and filtered using analog circuitry. The combination of digital noise shaping and analog filtering reduces the quantization noise to provide an accurate analog representation of the digital input signal.
Complicated linear analog filters are typically required at the output of the sigma-delta DAC to remove the out-of-band quantization noise. If these analog filters are not included then nonlinearities in the signal processing path following the sigma-delta DAC cause the out-of-band noise to intermodulate back into the passband. This reduces the dynamic range of the sigma-delta DAC. A preferred sigma-delta DAC implementation reduces the complexity and difficulty of design of these analog filters by reducing the sensitivity of the design to nonlinearities in analog signal components, and by reducing the required degree of filtering.
Digital sigma-delta modulators require oversampled data which is typically provided by an interpolator. The DAC""s input data may be provided at the Nyquist rate and typically requires interpolation to a higher rate before being proficed to the sigma-delta modulator. The sigma-delta modulator is clocked using a high-speed digital clock, which is typically much faster than the input data rate provided to the interpolator. In general, noise present near multiples of these clock frequencies does not affect the analog output signal, because these frequencies are well beyond the passband of the signal of interest. However, this noise has a tendency to inter-modulate back into the passband when nonlinearities are present in the signal processing path following the sigma-delta DAC.
An exemplary prior art DAC 100 that uses a single bit sigma-delta modulator 142, is discussed with reference to FIG. 1. DAC 100 includes generally an interpolator 141, a sigma-delta modulator 142, and a mixed-mode digital/analog finite impulse response (FIR) filter 143. Interpolator 141 has an input terminal for receiving digital input data 102, a first clock input terminal for receiving a clock signal labeled xe2x80x9cCLK1xe2x80x9d, a second clock input terminal for receiving a clock signal labeled xe2x80x9cCLK2xe2x80x9d, and an output terminal for providing an n-bit output signal 104. Interpolator 141 receives digital input data 102 at a first sampling rate (the frequency of CLK1), and provides the n-bit output code 104 at the output thereof at a second, higher sampling rate (the frequency of CLK2) by performing an interpolation between the samples. Input data 102 may already be oversampled as well.
Single bit sigma-delta modulator 142 has an input terminal connected to the output terminal of interpolator 141, a clock input terminal for receiving signal CLK2, and an output terminal for providing output signal 106. Sigma-delta modulator 142 shapes the quantization noise in output signal 106 out-of-band, and thus, output signal 106 is a substantially accurate digital representation of input data 102 within the passband.
FIR filter 143 has an input terminal connected to the output terminal of sigma-delta modulator 142, and an output terminal for providing output signal 108. FIR filter 143 is single-bit, m-stage mixed-mode analog/digital filter that includes m delay elements each with a corresponding analog weighting. Representative delay elements 151, 152, and 153, and representative weighting amplifiers 161, 162, and 163 are illustrated in FIG. 1. Each delay element, labeled xe2x80x9czxe2x88x921xe2x80x9d, has an input terminal connected to a previous delay element, if any, and an output terminal. Amplifiers 161, 162, and 163 have inputs connected to outputs of corresponding delay elements 151, 152, and 153, outputs connected to corresponding positive inputs of a summing device 170, and multiply the inputs thereof to implement FIR filter coefficients labeled xe2x80x9ch1xe2x80x9d, xe2x80x9ch2xe2x80x9d, and xe2x80x9chmxe2x80x9d, respectively, associated therewith. The coefficient tap weights h1, h2 . . . hm can be set to give a specified frequency response. Analog summing device 170 sums the outputs of all the amplifiers and has an output terminal for providing analog output signal 108 therefrom. Further details of this prior art sigma-delta DAC 100 are described in U.S. Pat. No. 5,323,157, entitled xe2x80x9cSigma-Delta Digital-to-Analog Converter with Reduced Noisexe2x80x9d (Ledzius et al.), which is incorporated herein by reference.
Conventional systems, such as the above described prior art DAC 100 suffer from numerous disadvantages. For example, single bit output 106 of sigma-delta modulator 142 has relatively high quantization noise before being filtered by analog FIR filter 143. Therefore, any noise on a reference signal, which could couple from clock frequencies and other high frequency signals, could easily modulate the quantization noise into the signal band.
For example, when the amplitude of the AC component of the digital input signal 102 is small, the quantization error signals within sigma-delta modulator 142 will tend to become periodic; that is, xe2x80x9cidle tonesxe2x80x9d or xe2x80x9climit cyclesxe2x80x9d are produced. This is a well-known characteristic of sigma delta modulators. Detailed explanation of idle tones and limit cycles is, for example, provided in the book xe2x80x9cDelta-Sigma Data Convertersxe2x80x94Theory, Design and Simulation,xe2x80x9d Norsworthy et. al., IEEE Press, Piscataway, N.J. (1997), which is incorporated herein by reference. The idle tones with largest amplitude are typically produced at frequencies close to one-half of the sample frequency of sigma-delta modulator 142. There are two common causes for these idle tones to fold into the signal passband and degrade the performance of the sigma-delta modulator. One cause is due to interfering digital signals which couple onto the reference voltage for sigma-delta DAC 100. When these interfering digital signals couple onto the reference voltage they intermodulate with the high-frequency quantization noise and cause it to fold into the passband. For this reason sigma-delta DAC 100 is sensitive to coupled digital noise. The second common cause of for the idle tones to fold into the signal passband is due to nonlinearities in the signal processing path at the output of sigma-delta modulator 142. In the presence of these nonlinearities the idle tones typically intermodulate with each other, causing them to fold into the signal passband. Accordingly it would be desirable to filter out the quantization noise related to tones near fs/2.
Another disadvantage of the above described prior art is that the clock frequency of analog FIR filter 143 is the same as the clock frequency of the sigma-delta modulator 142 (i.e., CLK2). Since modulator 142 is highly oversampled in typical applications, analog FIR filter 143 (e.g., including analog summer 170) must operate at a very high clock frequency. Since it typically takes less power and design complexity to operate analog circuitry at lower frequencies, it would be desirable to lower the frequency at which filters and other circuitry must operate.
The present invention is directed to a sigma-delta digital-to-analog converter (DAC). The sigma-delta DAC, according to an embodiment of the present invention, includes a digital-sigma delta modulator, a decimation filter, and a multi-bit DAC. The digital sigma-delta modulator receives a digital input signal and produces a quantized digital signal therefrom. The decimation filter receives the quantized digital signal and produces a decimated digital signal therefrom. The multi-bit DAC receives the decimated digital signal and produces an analog output signal therefrom. The analog output signal is representative of the digital input signal. As will be explained in more detail below, among other things, the decimation filter lowers the frequency at which filters and other circuitry (following the decimation filter) must operate.
In an embodiment of the present invention, the digital sigma-delta modulator includes a 2-level quantizer and the quantized digital signal is a single-bit digital signal. In another embodiment, the digital sigma-delta modulator includes a p-level quantizer (where p is an integer greater than 2), and the quantized digital signal is a multi-bit digital signal.
In an embodiment of the present invention, the decimation filter includes a multi-tap finite impulse response (FIR) filter (e.g., 3-tap digital FIR filter) and a decimator. The multi-tap FIR filter filters the quantized digital signal to produce a filtered signal including a sequence of sample values. Each sample value includes a plurality of bits. The decimator produces the decimated digital signal based on the sequence of sample values of the filtered signal.
According to a specific embodiment, the decimator discards one of every two sample values in the sequence of sample values of the filtered signal to produce the decimated digital signal. In this embodiment, the decimated digital signal has a sample frequency that is one half of a sample frequency of the filtered signal.
In an embodiment of the present invention, the multi-bit DAC includes k elements to be driven by a k-bit digital signal and produce k analog values therefrom, where the k-bit digital signal is representative of the decimated digital signal. A sum of the analog values from the k elements is representative of the digital input signal. Each of the k elements can be a current source that produces a respective one of the k analog values, with each of the k analog values being a current. In another embodiment, each of the k elements is a capacitor that produces a respective one of the k analog values, with each of the k analog values being a charge. Alternatively, each of the k elements is a resistor that produces a respective one of the k analog values, with each of the k analog values being a current. In an embodiment, the muti-bit DAC is a calibrated DAC.
According to an embodiment of the present invention, the k-bit digital signal is the decimated digital signal.
In an embodiment of the present invention, the decimated digital signal is a binary signal and the k elements of the multi-bit DAC are binary weighted elements.
In an embodiment of the present invention the multi-bit DAC includes a coder to receive the decimated digital signal and produce the k-bit digital signal therefrom. For example, if the decimated digital signal is an n-bit signal, the coder receives the n-bit decimated digital signal and produces the k-bit digital signal therefrom, where k may or may not equal n. The n-bit decimated digital signal can be a binary signal. In such an embodiment, the coder can be a binary-to-thermometer coder that converts the n-bit binary decimated digital signal to the k-bit digital signal, the k-bit digital signal being a thermometer coded signal.
In an embodiment of the present invention, the multi-bit DAC includes a mismatch shaping network to receive the decimated digital signal and produce the k-bit digital signal therefrom. In this embodiment, the k-bit digital signal is a shuffled signal.
In an embodiment of the present invention, the multi-bit DAC includes a coder and a mismatch shaping network. The coder receives the decimated digital signal and produces a coded signal therefrom. Then the mismatch shaping network receives the coded signal and produces the k-bit digital signal therefrom. The k-bit digital signal in this embodiment is a shuffled signal. In an embodiment, each of the k elements are substantially equally weighted.
According to an embodiment of the present invention, the multi-bit DAC includes an N-tap delay line and N multi-bit sub DACs, where N is greater than one. The N-tap delay line includes N multi-bit delay elements. A first of the N multi-bit delay elements receives a multi-bit digital signal representative of the decimated digital signal and produces a delayed multi-bit output signal therefrom. Each of the other N multi-bit delay elements receives a delayed multi-bit output signal from an immediately preceding one of the N multi-bit delay elements and produces a respective delayed multi-bit output signal therefrom. Each multi-bit sub-DAC is driven by a respective one of the delayed multi-bit output signals produced by a corresponding one the N multi-bit delay elements and produces an analog output therefrom. A sum of the analog outputs from the N multi-bit sub-DACs is representative of the digital input signal. In an embodiment of the present invention, each multi-bit sub-DAC includes k elements to be driven by a respective one of the delayed multi-bit output signals and produce k analog values therefrom. A sum of the analog values from the k elements corresponds to an analog output from one of the N multi-bit sub-DACs.
In an embodiment of the present invention, the multi-bit digital signal received by the first of the N multi-bit delay elements is the decimated digital signal. In this embodiment, the decimated digital signal can be, for example, a binary signal and each multi-bit sub-DAC can include binary weighted elements.
In an embodiment of the present invention, the multi-bit DAC further includes a coder to receive the decimated digital signal and produce the multi-bit digital signal therefrom. In this embodiment, the decimated digital signal can be an n-bit signal. The coder receives the n-bit decimated digital signal and produces the multi-bit digital signal therefrom. In this embodiment, the multi-bit digital signal being a k-bit digital signal, where k may or may not equal n. The n-bit decimated digital signal can be, for example, a binary signal. In such all embodiment, the coder can be a binary-to-thermometer coder that converts the n-bit binary decimated digital signal to the k-bit digital signal, the k-bit digital signal being a thermometer coded signal. Each multi-bit sub-DAC may include elements, with each of the k elements being substantially equally weighted.
According to an embodiment of the present invention, the multi-bit DAC further includes a mismatch shaping network to receive the decimated digital signal and produce the multi-bit digital signal therefrom. In this embodiment, the multi-bit digital signal is a k-bit shuffled digital signal.
In an embodiment of the present invention, the multi-bit DAC further includes a coder and a mismatch shaping network. The coder receives the decimated digital signal and produces a coded signal therefrom. The mismatch shaping network receives the coded signal and produces a k-bit shuffled digital signal therefrom. The k-bit shuffled digital signal is the multi-bit digital signal received by the first of the N multi-bit delay elements.
In an embodiment of the present invention, each multi-bit sub-DAC includes k elements that are each substantially equally weighted. Each sub-DAC may also include a coder and/or a mismatch shaping network.
The present invention is also directed to methods for converting a digital signal to an analog signal.